bug in mallocForeignPtrBytes (both 6.4 and 6.6)

[Bulat Ziganshin]

Hello Duncan,

Sunday, September 24, 2006, 12:22:38 AM, you wrote:

>> after program prints "40 mb allocated" look at Task Manager indication
>> - it shows that two times more memory actually in use. it seems that
>> problem is only with allocating memory buffers whose sizes are powers
>> of 2 or very close - bug shows for bufsize = 4kb, 1mb or 4095, but not
>> not for 4000 or 10^6

> The reason for this is that for larger objects ghc has an allocation
> granularity of 4k. That is it always uses a multiple of 4k bytes.

with 512k blocks it also allocates two times more data than requested.

> However a byte array has some overhead: it needs one word for the heap
> cell header and another for the length. So if you allocate a 4k byte
> array then it uses 8k. So the trick is to allocate 4k - overhead. This
> is what the Data.ByteString library does.

thank you - it will be ok for my lib


-- 
Best regards,
 Bulat                            mailto:Bulat.Ziganshin at gmail.com